The video showed a letter, said to have been written by Leading Seaman Turney, who is from Shrewsbury, to her parents in which she admitted that the navy personnel had "apparently" crossed into Iranian waters.

The pictures also featured the 14 servicemen seized along with Leading Seaman Turney.

Foreign Secretary Margaret Beckett said: "I am very concerned about these pictures and any indication of pressure or coercion of our personnel who were carrying out a routine operation in accordance with international law and under a United Nations Resolution."

Defence Secretary Des Browne said it was "completely unacceptable to parade our people in this way".

Former RAF pilot John Nichol, who was put on television by the Iraqis after being shot down in the first Gulf War, told BBC Radio 4's Today programme he had "no doubt" Leading Seaman Turney had been told what to say.

"It is a very difficult position - and she's in an unenviable position where she has to try to placate the people that have captured her and try to keep the whole group safe," he said.

GPS evidence

Iran has not disclosed where the service personnel are being held but the BBC has been told they are at a military base in Tehran.

Tony Blair has said it is time for the UK to "ratchet up" pressure on Iran.

Deputy Chief of the Defence Staff, Vice Admiral Charles Style, has produced data which he said proved the boarding party was 1.7 nautical miles inside Iraqi waters.

The Ministry of Defence also released a photograph of a handheld global positioning satellite device in HMS Cornwall's Lynx helicopter as it flew over the merchant vessel which was searched by the group before they were seized.

Ed Owen, who was an advisor to Jack Straw during his time as foreign secretary, told BBC Radio Five Live he thought Iran had deliberately engineered the incident.

"There are people in the Iranian regime who believe that the way to address what they regard as the dangers of the West, as well as expanding Iranian influence, is to confront the West," he said.

The Iranian embassy in London said the Britons had been 0.5 km inside Iranian territorial waters.